The present invention relates to a rotation-angle detector for detecting a rotation angle of, e.g., a steering shaft of a car.
Recently, cars have been provided with a variety of functions and various controls are thus required. For that purpose, cars are equipped with rotation-angle detectors car for detecting a rotation angle of the steering shaft.
A conventional rotation-angle detector is described hereinafter with reference to FIG. 11 through FIG. 14B. FIG. 11 is a perspective view of a conventional rotation-angle detector. FIG. 12 is a sectional view of the detector shown in FIG. 11. FIGS. 13A and 13B show voltage-waveforms with respect to a rotation angle detected by the detector shown in FIG. 11. FIGS. 14A and 14B show enlarged voltage-waveforms shown in FIGS. 13A and 13B.
In FIG. 11 and FIG. 12, rotary body 501, on whose rim rotary-gear 511 is formed, engages first detecting gear 502 and second detecting gear 503, and gears 502 and 503 have different numbers of cogs.
The numbers of cogs of gears 502 and 503 are set such that the numbers have periodicity with respect to a given rotation-angle of rotary body 501. In other words, when rotary body 501 rotates twice counterclockwise or clockwise from a null position, rotary body 501 is to return to an original engaging relation at the null position.
Inside the rim of rotary body 501, two engaging sections 512 are provided for engaging the steering shaft (not shown) extending through the center of rotary body 501. Gears 502 and 503 have magnets 504 and 505 at their centers.
Circuit-board 506 is placed above gears 502 and 503, and board 506 is equipped with first angle-sensor 507 and second angle-sensor 508 facing, respectively, the centers of gear 502 and gear 503. Further, board 506 has detecting circuit 509 comprising a micro-processor for processing detection signals supplied from sensors 507 and 508.
An operation of the rotation-angle detector discussed above is described hereinafter. In FIG. 11, when the steering shaft rotates, the torque is transmitted to rotary body 501 via the two engaging sections 512, so that rotary gear 511 rotates. The rotation of gear 511 causes gears 502 and 503 engaged with gear 511 to rotate, respectively. At this time, sensors 507 and 508 detect magnetism from magnets 504 and 505, whereby the rotation angles of gears 502 and 503 are detected.
Detection signals detected by sensors 507 and 508 are shown in FIGS. 13A and 13B. Since gears 502 and 503 have different numbers of cogs, the detection signals draw voltage-waveforms having rather different phases from each other with respect to their rotation angles while rotary body 501 rotates twice either clockwise or counterclockwise.
As the enlarged voltage-waveform in FIG. 14 shows, detecting circuit 509 detects voltages of sensors 507 and 508 (i.e., they are the detection signals) and calculates the voltages as well as the numbers of cogs of gears 502 and 503, so that rotation angle xcex8 of rotary body 501 is detected.
However, the conventional detector discussed above has encountered the following problems:
(a) The rotation angle of rotary body 501 is detected based on detecting the rotations of detection gears 502 and 503 engaged with rotary gear 511. Therefore, gaps between gears 511 and 502, as well as gaps between gears 511 and 503, and looseness due to these gaps cause an error in detecting an angle, particularly when the gears start rotating or rotate in a reverse direction.
(b) Detecting a rotation angle requires complicated calculations by detecting circuit 509.
(c) When a large rotation angle is detected, respective gears 511, 502 and 503 need to have greater numbers of cogs due to a periodic relationship between gear 511 and gears 502, 503. As a result, larger diameters are necessary, which prevents the entire detector from being downsized.
The present invention addresses the problems discussed above, and aims to provide a downsized rotation-angle detector that produces a smaller detection error, and a simpler calculation is carried out in a detecting circuit thereof.
The rotation-angle detector of the present invention comprises the following elements: a rotary body having a rotary gear; a change gear engaged with the rotary gear; a driven body moving following the rotation of the change gear, a first detector for detecting a movement of the driven body; a detecting gear engaged with the change gear; a second detector for detecting a rotation of the detecting gear; and a detecting circuit for processing detection signals supplied from the first and the second detectors. The first detector detects the movement of the driven body as a first detection signal gradually increasing or decreasing, and the second detector detects the rotation of the detecting gear as a second detection signal repeating continuously. The detecting circuit detects a rotation angle of the rotary body based on the two detection signals. This structure allows the detector of the present invention to produce a smaller error, be downsized, and make simple calculation in the detecting circuit.